Gear lubricant improving agents



United States Patent ()fifice Patented Dec. 19, 1961 The present invention relates, as indicated, to compositions intended for use in preparing lubricants for relatively moving metal surfaces. More particularly, it relates to compositions intended for the compounding of lubricants for the gears of automotive vehicles.

The problems of lubricating automotive gears became increasingly acute during the latter part of the decade l92(l1930, when manufacturers began to replace spiral bevel gears with hypoid gears in the rear axles of automotive equipment.

For reasons inherent in their design, hypoid gears permit the lowering of the center of gravity and the overall height of vehicles. Coupled with these desirable features, however, were many new lubrication problems. It was found that hypoid gears, unlike spiral bevel gears, could not be lubricated satisfactorily with ordinary mineral oils. The great pressures existing between the contacting metal surfaces in hypoid gear systems literally squeezed out the mineral oil lubricant, resulting in metal-to-metal contact under extremely high pressures and subsequent deformation and destruction of the gear surfaces.

Early workers in the lubricant art discovered that destructive metal-to-me-tal contact in hypoid gear systems could be avoided by the addition of certain oil-soluble chemical compositions to the mineral oil lubricant. These compositions, which were predominantly organic in nature but contained one or more inorganic elements such as sulfur, chlorine, and heavy metal in chemical combination, were believed to react with the gear surfaces under the elevated temperatures which generally accompany high pressures to form thin films or layers of iron sulfide, iron chloride, etc. on the gear surfaces. Such films acted as separatants and lubricants for the gear surfaces, thereby preventing destructive metalo-metal contact.

In 1945 two full-scale gear lubricant performance tests were promulgated by the Ordnance Department of the U.S. Army;-viz, specification AXS-l569, Test Procedure for Determining the Load-Carrying Characteristics of Universal Gear Lubricants Under Conditions of High Speed, and specification AXSlb70, Procedure for Deter-mining the Load-Carrying, Wear, Stability, and Corrosion Characteristics of Universal Gear Lubricants Under Conditions of High Torque and Low Speed. these two tests were originally intended to aid the government and, more particularly, the armed service branches, in purchasing gear lubricants, they were eventually adopted by the petroleum industry at large as criteria for the selection and purchase of gear lubricants destined for the civilian market. For a number of years these tests satisfied the need for tests of good reproducibility which would simulate in the laboratory, insofar as possible, service conditions encountered in actual use of all types of automotive equipment.

However, Within recent years, the sharp upward trend Although of engine horsepower has placed increasingly severe demands upon the hypoid gears which ultimately transmit the engine power to the drive Wheels. These demands are not met with a satisfactory margin of safety by currently available gear lubricants which will pass one or both of the Ordnance Department tests referred to above.

From an engineering standpoint, the new service requirement encountered is principally one of greatly in creased pressures or loads, particularly those of a sudden or shock nature, upon the gear teeth. Since the metallurgy and physical size of the gears has not been altered significantly to accommodate the greater engine torque, the lubricant alone must bear the heightened severity of operating conditions if it is to prevent gear failures. This need for a more satisfactory lubricant is particularly critical during the initial brealein period of a vehicle, for it is then that the gears are most susceptible to deformation.

As more or less a stop-gap measure, manufacturers of automotive equipment have factory-installed lubricants principally from the fact, well known to WOllCfiI'S in the lubricant art, that materials which help the lubricant perform its task under one set of operating conditions, say shock loading at speeds, generally reduce or, in some instances, even nullify the effect of materials added to improve the performance of the lubricant under a difierent set of operating conditions; e.g., high torque loading at relatively lower speeds.

By effecting a critical balance between the proportions of materials known to be eifective under (a) high speed operating conditions, and ([7) high torque relatively lower speed operating conditions,'prior Workers in the lubricant art have been able to produce lubricants which meet the requirements of the aforesaid Army Ordnance high speed and high torque performance tests.

All of such carefully balanced or compromise lubricants have not proved amenable to modification, however, so as to enable them to withstand both severe shock loads at high speeds and high torque loads at relatively lower speeds. The addition thereto of materials known to be effective under shock load conditions upsets the carefully balanced lubricant and rendersit substantially less effective in a high torque-low speed operating environment.

It is the principal object of the present invention therefore to provide novel improving agents for the production of lubricants which will render satisfactory performance the hypoid gear systems of modern, high-powered, automotive equipment under severe operating conditions characterized by shock load, high speed, and high torque demands.

This and related objects of the invention willbecome apparent as the description of the invention proceeds.

In accordance with the present invention, it has been discovered that this and other objects may be achieved by the use of lubricant improving agents which contain certain critical proportions of both (a) and oil-soluble zinc salt of a phosphorodithioic acid and (b) and oil-soluble organic polysulfide.

In a more particular sense, the invention relates to the provision of a liquid, homogeneous improving agent which, when present in gear lubricant compositions, increases the ability thereof to function under high speedshock load as well as high torque conditions of operation, consisting of the combination of:

A. an oil-soluble zinc salt of a di-alkyl ester of a phosphorodithioic acid in which the alkyl groups contain from 1 to about 40 carbon atoms; and

B. an oil-soluble organic polysulfide having at least 1 sulfur atom bonded only by secondary valence bonds, said polysulfide selected from the group consisting of di-benzyl polysulfides and chlorine: substituted dibenzyl polysulfides;

said components A and B being present in relative proportions such that when an admixture thereof is dissolved in a solvent therefor in an amount equal to from about 3.5% to about 15% based on the weight of the total solution, said solution will have contributed thereto by said components:

(i) from about 0.20% to about 0.75% phosphorus; and

(ii) from about 0.25% to about 1.0% of chemically combined sulfur which is bonded only by secondary valence bonds.

In addition to the two characterizing components A and B, the lubricant improving agent and lubricants compounded therefrom may also contain, if desired, auxiliary improving agents such as, for example, rust inhibitors, oiliness agents, viscosity index improvers, and the like. The presence of these and other auxiliary agents, however desirable they may be for their contribution to the qualities of the finished lubricant, is not required to prevent gear deformation under severe operating conditions.

COMPONENT A: THE OIL-SOLUBLE ZINC SALT OF A DI-ALKYL ESTER OF A PHOSPHORO- DITHIOIC ACID The components of this type may be defined as zinc salts of phosphorodithioic acids having the structure in which R and R are alkyl radicals each containing from 1 to about 40 carbon atoms. These alkyl radicals may be straight chain or branched, and they may be alike or dissimilar. Thus the zinc salt of component A may be the zinc salt of a simple di-ester, i.e., one in which the alkyl radicals are alike; or it may be the zinc salt of a mixed di-ester, i.e., one in which the alkyl radicals are dissimilar; it may also be the zinc salt of a mixture of different simple di-esters, e.g., the zinc salt of a mixture of di-isopropyl phosphorodithioic acid and di-n-hexyl phosphorodithioic acid; or it may be the zinc salt of a mixture of a simple di-ester and a mixed di-ester; and lastly it may be the zinc salt of a mixture of mixed diesters.

The character of R and R in the structural formula s illustrated by the following examples: methyl, ethyl,

n-propyl, isobutyl, n-amyl, tert-amyl, 2-methyl, pentyl-4, 2-ethyl hexyl n-octyl, nonyl, decyl, dodecyl, tetradecyl, octadecyl, eicosyl, tricosyl, and others having up to about 40 carbon atoms.

A particularly preferred species of component A is the zinc salt of a di-alkyl ester of a phosphorodithioic acid having the structure in which R contains at least six carbon atoms and R contains less than six carbon atoms. Another preferred species for use as component A is the zinc salt of a mixture of different di-alkyl esters of a phosphorodithioic acid, one of said di-alkyl esters containing only radicals having less than six carbon atoms and another of said di-alkyl esters containing only radicals having at least six carbon atoms. In each of these preferred species an especially valuable subspecies is one in which the lower molecular weight alkyl group is the isopropyl radical and in which the higher molecular Weight alkyl group is the 2-methyl-pentyl-4 radical. These particular species and subspecies are disclosed in US. 2,838,555.

Other specific examples of compounds which are use ful as component A include zinc salts of the following:

Di-n-hexyl phosphorodithioic acid Di-n-octyl phosphorodithioic acid Di-dodecyl phosphorodithioic acid Ethyl octyl phosphorodithioic acid n-Propyl octyl phosphorodithioic acid Isobutyl decyl phosphorodithioic acid Isoamyl n-hexyl phosphorodithioic acid Methyl octadecyl phosphorodithioic acid The preparation of the phosphorodithioic acids from which the zinc salts of component A may be prepared are readily available by the well known process involving the reaction of an alcohol with phosphorus pentasulfide.

COMPONENT B: THE OIL-SOLUBLE ORGANIC POLYSULFIDE HAVING AT LEAST ONE SUL- FUR ATOM BONDED ONLY BY SECONDARY VALENCE BONDS Materials useful as this component have been defined as those selected from the group consisting of di-benzyl polysulfides and chlorine-substituted di-benzyl polysulfides.

By the phrase sulfur atom bonded only by secondary valence bonds I mean a sulfur atom which is bonded only to one or more sulfur atoms in the polysulfide molecule and is thus devoid of valence bonds extending to the organic radicals in the polysulfide molecule. The exact nature of such secondary valence bonds is not clearly understood, but it is known that they are much weaker than the covalent bond which exists between a sulfur atom and a carbon atom of an organic radical in an organic polysulfide.

Due to the relatively Weak attachment of sulfur atoms which are bonded only by secondary valence bonds, such sulfur is more readily given up by the molecule; i.e., is more reactive chemically, than sulfur which is bonded to a carbon atom of an organic radical. The very fact that such sulfur is chemically reactive facilitates its determination. For example, a test sample of the organic polysulfide may be treated with reagents which are known to react with and thus to remove reactive sulfur such as; e.g., warm aqueous caustic solutions, warm aqueous solutions of metallic monosulfides, finely divided metals such as copper, lead, iron, silver, etc. The loss in sulfur content of the test sample of organic polysulfideafter such treatment corresponds to the amount of reactive sulfur originally present; i.e., that sulfur which is bonded only by secondary valence bonds.

Thus it is possible to characterize an organic polysulfide both by its content of (a) total sulfur; i.e., sulfur in all forms, and (b) sulfur bonded only by secondary valence bonds. The discovery that certain. critical amounts of the kind of sulfur indicated in, (12) must be present in my lubricants constitutes one of the principal features of my invention.

The following partial structures illustrate some of the many arrangements which sulfur atoms can assume in organic polysulfides. In each of these partial stmctures the presence of an asterisk next to a sulfur atom indicates that such sulfur atom is held in the molecule by secondary valence bonds only:

Fromv a study of the structures given above, it. will be apparent, that organic polysulfides of like molecular weight and containing the samepercentages of chemical elements may possess widely different amounts of reactive sulfur depending, on the mode of attachment of the sulfur atoms within the molecule, Those structures which possess the largest number of sulfur atoms bonded only by secondary valence bonds will possessthe highest percentage of reactive sulfur.

Specific examples of organic polysulfides which contain at least one sulfur atom bonded. only by secondary valence bonds and which are useful as component B in lubricants of this invention are, for example:

Di-benzyl trisulfide Di-benzyl tetrasulfide Di-benzyl polysulfides containing an average of 4.6 sulfur atoms per molecule Di-benzyl pentasulfide Di-benzyl hexasulfide Monochloro di-benzyl tetrasulfide Di-(4-chlorobenzyl)tetrasulfide Bis-(2,4-dichlorobenzyl) tetrasulfide The preparation of materials useful as component B may be accomplished by any of the many different processes which are known and disclosed in the art including, for example, the reaction of halogen-bearing organic compounds with alkali metal polysulfides, the reaction of mercaptans with sulfur and/ or sulfur halides, the reaction of saturated and unsaturated hydrocarbons with sulfur and/ or sulfur halides, the reaction of organic monosulfides with sulfur, etc.

Auxiliary improving agents As previously indicated, there may also be present in my lubricants certain auxiliary improving agents such as, for example, detergents, rust-preventatives, film strength agents such as halogenated organic compounds; oiliness agents such as fatty oils and sulfurized fatty oils, pour point depressors, foam inhibitors, viscosity index improvers, oxidation inhibitors, odor improvers and the like. Included among the many materials useful for these various purposes are the following:

Detergents such as metal salts of petroleum naphthenic acids, petroleum sulfonic acids, the higher fatty acids, etc.; rust-preventatives such as basic metal petroleum sulfonates, metal phenolates, organic amines, benzyl thiocyanate, etc; film strength agents such as chlorinated paraflin waxes containing from 20 to 70% chlorine, chlorinated eicosane containing from 50 to 60% chlorine, hexachloro-diphenyl ether, polychlorobiphenyl', etc.; oiliness agents such as olein, methyl oleate, oleic acid, sulfurized and non-sulfurized sperm oil, corn oil, etc.; pour point depressors such as Wax-alkylated naphthalene or phenanthrene, etc.; foam inhibitors such as the polymeric di-alkyl silicones, etc.; and viscosity index improvers such as polymerized and co-polymerized alkyl methacrylates, polymerized butylenes, etc.

It is to be understood, however, that the use of any of such auxiliary agents is optional and not required in our lubricants to prevent the deformation of gear surfaces.

The oil base The lubricating oil base in which the characterizing components A, and B of this invention and any desired auxiliary agents are incorporated may be. of synthetic, vegetable, animal, or mineral origin. Because, of their "low cost, availability, and desirability, the mineral: oils;

i.e., those derived from petroleum, find the widest application in the lubricant art. 7

There are at the present time sundry mineral oils, each best suited from the standpoint of viscosity and other properties for different climates and operating environ.- ments. The oil base of a lubricating composition of the present invention will preferably comprise a mineral lubricating oil having characteristics now well recognized as best suited for such environment and climate.

In Table'I following, the characteristics of mineral lubrieating oils best suited for a given environment and-climate are disclosed. The actual upper limit of preferred; viscosity index isinfinite for most uses. The values given in the table for viscosity index. represent current. commercial maximum values.

TABLE I Application in- Type of climate Automotive Industrial gears gears Arctic: Degrees F. Degrees F.

Preferred viscosity range 1 30-80/210 30-1,000/2l0 Flash point. preferably no lower than 300 300 Pour point preferably no higher than 30 10 Temperate:

Preferred viscosity range 1 50-140/210 50-2,00fi/2l0 Flash point preferably no lower than 325 325 Pour point preferably no higher than 20 Tropical:

Preferred viscosity range 1 30-200/210 S02,000/2l0 Flash point preferably no lower than" 350 325 Pour point preferably no higher than 20 30 Preferred viscosity index for all climates (Dean and Davis scale) 75-150 3.3120

l Expressed in Saybolt Universal seconds at the indicated temperature,

Lubricants and lubricant improving agents As indicated previously, a number of hypoid gear lubricants representative of the several fundamental types which are commercially available were first investigated to determine their effectiveness in preventing deformation of gear surfaces in an operating environment characterized by severe shock loads at high speeds and high torque loads at relatively lower speeds.

Full-scale performance tests; i.e., tests using standard automative equipment, employed in this connection included the Army Ordnance AXS1569 high speed and AXS-1570 high torque tests and, in addition, a new severe shock test using a 1953 model Buick passenger car equipped with a torque converter transmission of the Dynaflow variety. I

The latter test, termed for convenience the Buick Shock Test," is one of almost unbelievable severity. It is carried out in the laboratory by mounting the rear drive wheels of the car on a heavy shaft which is free to revolve and which is designed to simulate the inertia of the car on a level highway. Although the test could conceivably be conducted on the open road, the severe operating conditions specified in its procedure would make such a course extremely hazardous.

In carrying out the test, the rear axle of the car is filled to the specified level with the desired test lubricant and the car is run for the equivalent of 19 miles at 40-50 mph. to break in the new gears. The car is then accelerated from 0 to 70 mph. and decelerated to 0 mph. with the transmission in drive position. If no unusual noise is detected in the rear axle, the car is subjected to a high speed evaluation comprising cycles of acceleration and deceleration, 0 to 70 to 0 m.p.h., with the transmission in drive position.

Next, for the shock portion of the test, the car brakes are applied, the throttle opened with the transmission in low position, and the brakes released suddenly. When the car reaches 30 mph. the transmission is shifted to the drive position and acceleration is continued to 75 mph. The throttle is then closed and the car is decelerated to 65 mph, at which speed the transmission is shifted abruptly to the low position. From the preceding description, it is apparent to one familiar with automotive equipment that shook loads of extreme severity are placed upon the coast and drive surfaces of the hypoid gear teeth.

After the shock portion of the test is completed, the high speed evaluation described earlier and comprising 10 cycles of acceleration and deceleration is repeated.

The car is stopped and the pinion and ring gears are removed and inspected for evidence of deformation such as rippling, ridging, spalling, and scoring. These terms define certain specific kinds of deformation which are readily distinguished from each other by trained mechanics. If such inspection shows the gear teeth to be free from deformation, the lubricant is said to have passed the test.

Table II includes the results obtained on commercially available hypoid gear lubricants in the three full-scale tests which have been described. The lubricants are defined by chcmical type in terms which are familiar to those in the lubricant art. It will be noted that none of the lubricants gave satisfactory performance in all of the tests, even though several satisfied the less rigorous requirements of the two well-known Army Ordnance tests.

These results point up the critical situation which preceded the present discovery; viz, that no hypoid gear lubricant was available which would withstand both the rigors of shock loads at high speeds and high torque loads at relatively lower speeds.

As stated hercinbefore, lubricants capable of giving satisfactory performance under these extremes of operating conditions can be made by incorporating therein certain critical proportions of the two characterizing components; viz, (A) an oil-soluble zinc salt of a di-alkyl phosphorodithioic acid and (B) an oil-solub1c bcnzyl or chlorobenzyl polysulfide having at least one sulfur atom bonded only by secondary valence bonds.

Table III includes test data on many lubricants containing the characterizing components A and B of the present invention in amounts which either come within or fall outside of the ranges found to be critical for each.

The tests employed were the Buick Shock test, the Army Ordnance AXS-1570 high torque test, and the SAE Extreme-Pressure Lubricant test at 1000 r.p.m. Army Ordnance test AXS-1569 was not employed in these studies since it had been determined that any lubricant which passed the Buick Shock test, which test includes a high speed evaluation, would always pass the less severe AXS-1569 test.

The SAE Extrcme Prcssurc Lubricant test included in Table III is a test which is well-known in the lubricant art. It is carried out on the SAE Lubricant machine in the manner set forth on page 45 of A.S.T.M. Bulletin No. 181, April 1952. It was found that lubricants which failed this test; i.e., would not sustain the full 590-600 pounds load, also failed the Buick Shock test. Since it was not feasible to conduct Buick Shock tests on all of the large number of lubricants investigated, the SAE test was employed to screen out lubricants which would not pass the former test.

It will be noted that in every instance when one or both of the two characterizing components A and B was present in an amount falling outside of the critical range disclosed herein, the lubricant failed to give satisfactory performance. On the other hand, when these same components were each present in a lubricant in amounts within the range found to be operative, the lubricant was effective in preventing gear deformation under both shock loads at high speeds and high torque loads at relatively lower speeds. For convenient identification, lubricants of the invention in Table III have been indicated by an asterisk Additional examples of lubricants of the invention are disclosed in Table IV. These are presented for purposes of illustration only and are not to be construed as limiting 9 the scope of the invention, particularly with respect to the inclusion of optional auxiliary improving agents such as;- eg, detergents, oiliness agents, foam inhibitors; etc.

The numerical values inparentheses; e.g. (0.37% P) and (0.5 sec. S), following the characterizingcompoeral improving agents can be adjusted within the c oncen nents: A and B indicate, respectively, the percent phostrate by the manufacturer. Then in preparing the finished phorus and percent sulfur bonded only by secondary lubricant, asingle addition of onlyone concentrate need valence bonds imparted to thefinishcd lubricant; by the be made to the oil base to impart thereto the desired particular component. improving-agents in predetermined relative proportions.

TABLE III minimize the problems associated with handling, storage, and. transportation. A further. benefit in cases where two or more separate improving agents are tobe. incorporated in the oil base is that the relative proportions of the sev-.

[All percentages of lmproving agcnts are byweigh-t based on the totallubrieant} Compoenth materials employed:

A-l inc di-organoposphorodithioatemade bythe neutralizing a mixture of 40 mole-percent of di-isopropyl phosphorodithioic acid and 60 mole percent of d A2=ziuc di-( i-rnethyl-2pentyl) phosphorodithioate Component B materials employed:

Bl=r .onoehler dibenzyl tetrasulfide (4-methyl-zrpe ntyl) phosphoroditfioic acid with zinc oxide- B2=di-benzyl polysulfides containing an average 0114.6 sulfur atoms per molecule [SAE 90 grade gear lubricant containing the indicated kinds andamounts of improving agents] Component A Component B Lubricant N0. i i SAE Test AX'S 1570 Percent P Percent at 1000 Buick High: Kind Percent contrib- Kind Percent sec. S conr.p;m Shock Torque uted to tributed to lube lube 1 TABLE IV Lubri- Composition Percent by cant No.

weight Di-benzyl polysulfides containing an average of 4.6 sulfur atoms per molecule (0.68% sec. S). Sulfurizrd sperm oil. Chlorinated oicosane containing 50% chlorine. SAE 80 gear lubricant. Zine di-octyl phcsphorodithioate (0.6% P). Di-ehlorobenzyl trisulfide (0.5% sec. S). Octadccylanliue caprylate. Zinc di-butyl dithiocarbamate. SAE 90 gear lubricant. I Didodecyl phosphorodithioate (0.75% P). Di-benzyl hexasulfide (0.3% sec. S). Oleie acid. Lard oil. SAE 90 gear lubricant. Di-octyl phosphorodithieate (0.5% P). Di-bcnzyl tutrasulfide (0.7% sec. S). Basic calcium petroleum sullonate.

It is common practice in the lubricant additive industry to provide a liquid, homogeneous improving agent concentrate containing high percentages of one or more separate improving agents and, optionally, a proportion of a mineral oil, preferably one of low viscosity. Such concentrates dissolve readily in lubricating oil bases and Concentrates intended for compounding with an oil base to yield lubricants of the present invention will always contain the two previously identified characterizing components A and B and optionally:

(l) certain desired auxiliary improving agents; and

(2) a proportion, generally from about 10% to about by weight based on the total concentrate, of a lowviscosity mineral oil.

As previously indicated, components A and B will be present in such relative proportions in the concentrate that when the concentrate is incorporated in the lubricant, such lubricant will have contributed thereto by said components:

(1) from about 0.20% to about 0.75% and preferably from about 0.3% to about 0.5% phosphorus; and

(2) from about 0.25% to about 1.0% and preferably from about 0.5% to about 0.9% of chemically combined sulfur which is bonded only by secondary valence bonds.

Depending on the particular components A and B employed, the presence of auxiliary improving agents, and, in some instances the presence of mineral oil in the concentrate, the amount thereof required to impart to the 111- bricant the necessary percent phosphorus and percent sulfur bonded only by secondary valence bonds will vary considerably. Generally an amount of the concentrate equal to from about 3.5% to about 15% based on the Weight of the total lubricant will be suitable.

This application is a continuation-in-part of applicants copending application Serial No. 484,555, filed January 27, 1955, and now abandoned. 7

Other modes of applying the principle of the invention v may be employed, change being made regarding the de- 11 when present in gear lubricant compositions, increases the ability thereof to function under high speed shock load as well as high torque conditions of operation, consisting of the combination of:

A. an oil-soluble zinc salt of a di-alkyl ester of a phosphorodithioic acid in which the alkyl groups contain from 1 to about 40 carbon atoms; and

B. an oil-soluble organic polysulfide having at least one sulfur atom bonded only by secondary valence bonds, said polysulfide selected from the group consisting of di-benzyl polysulfides and chlorine-substituted di-benzyl polysulfides;

said components A and B being present in relative proportions such that when an admixture thereof is dissolved in a solvent therefor in an amount equal to from about 3.5% to about 15% based on the weight of the total solution, said solution will have contributed thereto by said components:

(i) from about 0.20% to about 0.75% phosphorus; and

(ii) from about 0.25% to about 1.0% of chemically combined sulfur which is bonded only by secondary valence bonds.

2. The improving agent of claim 1 characterized further in that component A is the zinc salt of di-alkyl ester of a phosphorodithioic acidhaving the structure contains less than six carbon atoms.

3. The improving agent of claim 1 characterized fur- 12 ther in that component A is the zinc salt of a mixture of different di-alkyl esters of a phosphorodithioic acid, one of said di-alkyl esters containing only radicals having less than six carbon atoms and another of said di-alkyl esters containing only radicals having at least six carbon atoms.

4. The improving agent of claim 1 characterized further in that component A is the zinc salt of di-(4-methyl- 2-penty1) phosphorodithioate.

5. The improving agent of claim 1 characterized further in that component B is a mixture of di-benzyl polysulfides in which the average number of sulfur atoms per molecule is 4.6.

6. The improving agent of claim 1 characterized further in that component B is monochlor di-benzyl tetrasulfide.

References Cited in the file of this patent UNITED STATES PATENTS 2,205,858 Mikeska et a1. June 25, 1940 2,208,161 Prutton et a1. July 16, 1940 2,261,047 Asseff Oct. 28, 1941 2,364,284 Freuler Dec. 5, 1944 2,382,115 Stucker Aug. 14, 1945 2,398,416 Denison et a1 Apr. 16, 1946 2,473,511 Denison et al June 21, 1949 2,514,625 Clausen et a1 July 11, 1950 2,689,220 Mulvany Sept. 14, 1954 2,838,555 Goldsmith June 10, 1958 FOREIGN PATENTS 679,465 Great Britain Sept. 17, 1952 679,466 Great Britain Sept. 17, 1952 689,759 Great Britain Apr. 1, 1953 

1. A LIQUID, HOMOGENEOUS IMPROVING AGENT WHICH, WHEN PRESENT IN GEAR LUBRICANT COMPOSITIONS, INCREASES THE ABILITY THEREOF TO FUNCTION UNDER HIGH SPEED SHOCK LOAD AS WELL AS HIGH TORQUE CONDITIONS OF OPERATION, CONSISTING OF THE COMBINATION OF: A. AN OIL-SOLUBLE ZINC SALT OF A DI-ALKYL ESTER OF A PHOSPHORODITHIOIC ACID IN WHICH THE ALKYL GROUPS CONTAIN FROM 1 TO ABOUT 40 CARBON ATOMS, AND B. AN OIL-SOLUBLE ORGANIC POLYSULFIDE HAVING AT LEAST ONE SULFUR ATOM BONDED ONLY BY SECONDARY VALENCE BONDS, SAID POLYSULFIDE SELECTED FROM THE GROUP CONSISTING OF DI-BENZYL POLYSULFIDES AND CHLORINE-SUBSTITUTED DI-BENZYL POLYSULFIDES, SAID COMPONENTS A AND B BEING PRESENT IN RELATIVE PROPORTIONS SUCH THAT WHEN AN ADMIXTURE THEREOF IS DISSOLVED IN A SOLVENT THEREFOR IN AN AMOUNT EQUAL TO FROM ABOUT 3.5% TO ABOUT 15% BASED ON THE WEIGHT OF THE TOTAL SOLUTION, SAID SOLUTION WILL HAVE CONTRIBUTED THERETO BY SAID COMPONENTS: (I) FROM ABOUT 0.20% TO ABOUT 0.75% PHOSPHORUS, AND (II) FROM ABOUT 0.25% TO ABOUT 1.0% OF CHEMICALLY COMBINED SULFUR WHICH IS BONDED ONLY BY SECONDARY VALENCE BONDS. 